1. Field of the Invention
The present invention relates to an abnormality diagnosis apparatus for an internal combustion engine capable of diagnosing the presence or absence of abnormality in the internal combustion engine in which a three-way catalytic converter (hereinafter simply referred to as a “catalyst”) for purifying an exhaust gas is arranged in an exhaust system, with a pair of air fuel ratio sensors being disposed at locations upstream and downstream of the catalyst.
2. Description of the Related Art
As a conventional abnormality diagnosis apparatus for an internal combustion engine, there has hitherto been known one in which a pair of air fuel ratio sensors are arranged at locations upstream and downstream of a catalyst, so that an abnormality in the catalyst is detected based on detection signals from the individual air fuel ratio sensors (see, for example, a first patent document: Japanese patent application laid-open No. H6-129240).
Also, there has been proposed an apparatus in which a pair of air fuel ratio sensors are arranged at locations upstream and downstream of a catalyst, and an abnormality in an air fuel ratio feedback control system is detected based on an oxygen sensor downstream of the catalyst (see, for example, a second patent document: Japanese patent application laid-open No. 2004-204716).
In addition, there has been proposed an apparatus in which the amount of oxygen storage during a fuel-cut operation is calculated so that an abnormality in an oxygen sensor downstream of a catalyst is diagnosed in accordance with the amount of oxygen storage (see, for example, a third patent document: Japanese patent application laid-open No. 2003-14683).
In the conventional abnormality diagnosis apparatuses for an internal combustion engine, in case of the first patent document, there is a problem that an abnormality in the catalyst can be determined, but an abnormality in the fuel control system can not be detected.
Also, in case of the second patent document, an abnormality in the feedback control system is detected by the oxygen sensor downstream of the catalyst, but no consideration is given to the state of the oxygen storage in the catalyst, and no account is taken of the case where the amount of feedback control by means of the oxygen sensor downstream of the catalyst is limited to a small value because of the amount of oxygen storage in the catalyst, or the case where the signal of the oxygen sensor downstream of the catalyst is output in a delayed manner, so there is a problem of possibility of making an incorrect abnormality determination.
Moreover, since no consideration is given to the state of the catalyst, there is a problem of possibility of making an incorrect abnormality determination when the output signal of the oxygen sensor downstream of the catalyst is disturbed or fluctuated due to deactivation or degradation of the catalyst.
In addition, in the third patent document, the amount of oxygen storage in the catalyst is calculated but only an abnormality in the oxygen sensor downstream of the catalyst is diagnosed, thus posing a problem that an abnormality in the fuel control system can not be detected.
Further, in the feedback control according to the signals of the oxygen sensors upstream and downstream of the catalyst, as in the conventional apparatuses described in the above-mentioned first through third patent documents, there arises the following problem. That is, in case where there occurs a situation where an amount of fuel more than estimated is injected due to the occurrence of abnormality in the fuel system, e.g., abnormality in a fuel injector only for one cylinder in a multicylinder internal combustion engine, it becomes impossible to control the exhaust gas downstream of the catalyst within a normal range because of the construction or configuration of each of the conventional apparatuses.